The invention relates to a semiconductor device comprising a CMOS image sensor as being recited in the preamble of claim 1. It is to be noted that in this application C-MOS image sensor means that the sensor is based on CMOS (=Complimentary Metal Oxide Semiconductor) technology or on NMOS technology or on PMOS technology. In CMOS technology both NMOS and PMOS technology is used.
Such sensors have been in use for imaging transmission patterns from ionizing radiation, that without limitation are used in medical diagnostics. A high charge capacity and a low noise are essential for such a sensor in general but in particular for said application. Prior art, in particular US Patent Application US 2005/0017245 to Manabe et al. that has been published on Jan. 27, 2005 uses CMOS image sensor having a so-called 4T pixel which means that it comprises a photodiode and four transistors, in this case a transfer transistor, a source follower transistor, a reset transistor, and a select transistor. The photodiode is pinned. That means that its structure is such that at a certain voltage, the so-called pin voltage (Vpin), the surface potential of the np-diode (i.e. the upper pn junction) is “pinned” to the p-well or p-substrate through the presence of a (thin) p+ region above the np-diode. Both the surface state shielding by pinning the photodiode and the low maximum electrical field in such a photodiode with two junctions (pnp structure) results in a low leakage current and low shot-noise. Such a device also has a very low reset noise provided that the diode is completely depleted after reset.
On the other hand, such a device has the disadvantage that its photodiode does not have a very high charge capacity. This is mainly related to the capacity limitation of the sense node in such a 4T configuration. Moreover, also the relatively large number of transistors, i.e. 4 pro pixel, requires more surface area and contributes to some extend to a lower charge capacity of the known device. In particular for the envisaged medical application of X-ray imaging, a high charge capacity for each pixel is very desirable. Furthermore in a 4T configuration a so-called nondestructive readout is not possible since the detected charge is necessarily transferred from the photodiode to a sense node and thus is lost for further detection. The present inventors have recognized that these disadvantages can be overcome in a manner implying that also the noise is low and a linear response is obtained.